Method and device for the successive zone melting and resolidifying of extremely pure substances



Feb. 8, 1966 K. SIEBERTZ 3,234,009

METHOD AND DEVICE FOR THE SUCCESSIVE ZONE MELTING AND RESOLIDIFYING 0F EXTREMELY PURE SUBSTANCES Original Filed Feb. 10, 1954 United States Patent Office 3,234,009 Patented Feb. 8, 1966 4 Claims. ((11.75-)

This is a division of my application Serial No. 409,420, filed February 10, 1954, now Patent No. 3,086,856.

It is known to liberate last traces of impurities from generally pure materials by succesively melting a zone of an elongated body or rod of the material in such a manner that the molten zone travels gradually from one end of the body to the other. Due to the concentration jump of the dissolved impurities in the border surface between the liquid and solid portions of the rod, the molten zone becomes enriched in impurities while a depletion occurs in the resolidifying material, or vice versa. Due to the fact that the melting zone is gradually displaced from one to the other end of the rod, the impurities also migrate toward one of these ends. If neces sary, the process can be repeated for obtaining maximum degrees of purity. At the same time, the process may serve to produce a single crystal from an originally amorphous or polycrystalline material, which may be sintered or otherwise produced. This requires placing into the melting zone, at the point where it commences its travel along the body, a single crystal of the proper orientation to serve as a crystal germ.

The zone-melting process as heretofore known requires placing the material to be melted into a crucible or boat of refractory material. The results of the process, therefore, are affected by the fact that the material being processed can take up new impurities from the surface of the crucible walls.

It is an object of my invention to avoid this deficiency and to improve the zone-melting method toward achieving a higher degree of purity of the material being treated and toward a reduction in the processing time needed for securing a given degree of purity.

To this end, and in accordance with a feature of my invention, the body to be successively zone melted is mechanically supported at only a few places so that the melting zones are only in the slightest degree, if at all, in contact with the means holding or containing the body. To prevent the molten material from running off due to its gravity, the melting zone is kept so narrow that the surface tension suffices for retaining it between the adjacent solid parts of the material. To achieve this result, and according to another feature of my invention, the elongated and preferably rod-shaped body is arranged vertically, in contrast to the horizontal arrangement heretofore customary.

According to another feature of my invention, the running-01f to the molten material is prevented by continuously revolving the body during the zone-melting operation.

According to still another feature of the invention, the body may be given a horizontal or inclined arrangement and is heated only from above so that the melting zone does not extend through the entire diameter of the rod but forms only a molten segment or cone-shaped zone comprising only about the upper half of the rod, while the lower part of the rod remains solid and carries the melting zone.

According to a further feature of the invention, the liquid material in the melting zone, which in this case may traverse the entire cross section of the rod, is kept freely floating by electromagnetic or pneumatic means. To this end, an electric field may be produced which, at the place of the melting zone, produces potential surfaces of a corresponding shape, for instance saddle shape, to magnetically support the material, the material being electrically conductive at least when in its molten condition. The field may be equipotential. The same effect can be obtained by the coaction of correspondingly arranged streams of gas. The gas to be used for this purpose is the same protective or inert gas that forms the atmosphere within which the entire zone-melting operation is to be performed.

The foregoing and other features of the invention will be more fully understood from the following description of the embodiments of devices according to the invention illustrated on the drawing, in which:

FIG. 1 illustrates an arrangement for heat-treating a vertically held elongated body of material according to the invention wherein the melted zone is prevented from running off by means of a plurality of inclined blowers;

FIG. 2 illustrates an alternative method of practicing the invention wherein the material being treated is horizontally or slantingly disposed;

FIG. 3 is a transverse cross-sectional View of FIG. 2 taken through the melted zone; and

FIG. 4 shows an apparatus for performing the method according to the invention with an arrangement as shown in FIG. 1.

According to FIG. 1, a rod 1, of aluminum for example, to be processed is held in vertical position by clamping devices 2 and 3 engaging the two ends of the rod. A directly or indirectly heated ring 4 of small axial width in comparison with the length of the rod 1 surrounds a zone 5 of the rod in concentric relation thereto. The whole device is located in a vacuum or within a protective inert gaseous medium. A number of nozzles 6 are distributed around the rod below the heater 4. Their blowing direction is such that the melting zone 5 is impinged on all sides from below so that the molten material cannot drop off. The gas supplied to the nozzle 6 is preheated by a heating device (not shown) to prevent marginal cooling of the melting zone. The clamps 2 and 3 together with the rod 1 are displaceable in the direction of the arrow 7 relative to the stationarily mounted heating and blowing devices 4, 6. The mechanism for moving'the holders together with the rodmay comprise a motor driven worm screw, a guide device for allowing simultaneous translatory motion of the two holders, and gear mechanism carried by the holders and in meshing engagement with the worm screw. During the operation, therefore, the melting zone 5 travels from the bottom upwardly along the rod.

In the embodiment shown in FIGS. 2 and 3, the rod 1 of the material to be purified is arranged horizontally. An arc lamp 8 serves as a source of heat. Its radiation is concentrated upon the surface of rod 1 by means of a hollow mirror 9 and produces a trough or cone-shaped melting zone 10. This melting zone is carried by the underlying solid portion of the rod. A blowing device 11 directs a cooling stream of protective gas against the bottom side of rod 1 to prevent the melting zone from breaking through the solid area. The rod 1 is continuously kept in revolution in the direction of the arrow 12 by means of a motor (not illustrated) adapted to turn a holding device at one end of the rod being processed. In addition, the radiation source 8, 9 is displaced relative to rod 1 in the direction of the arrow 13. This can be accomplished by having the heat source and blowers fixed on a support that is slidable in guide means coextensive with the rod, and by providing a motor of suitable speed having an output shaft cooperatively connected with the displaceable support. As a result, the melting zone is drawn through the rod from the left to the right on a helical path. FIG. 3 shows the heating and cooling means in relation to a cross section of rod 1 through the melting zone 10. As apparent from FIG. 3, the heat source 8, 9 is given a somewhat slanting arrangement while the cooling effect of the nozzle arrangement 11 extends over a larger peripheral range of the rod and is effective not only from below but also from the side of the rod.

In the device according to FIG. 4, the rod 1 to be processed is mounted at both ends in respective holders 2, 3 and is surrounded by a ring-shaped heater 4 for melting an axially narrowcross-sectional zone 5 as described above with reference to FIG. 1. Holder 3 is mounted on a support 21 which is slidably connected with a base plate 22 and can be displaced in the axial direction of rod 1 by means of a screw spindle 23. A bell-shaped housing 24 sealed against the base plate.22 encloses the rodl to permit operating in vacuum or in a protective atmosphere. The holder 2 is mounted on a shaft ZS-thatpasses through the housing to the outside and carries a gear 26 to be driven from a motor 27. The heater ring 4 is mounted on an axially displaceable rod 28 that extends to the outside of the housing 24 where it is connected by a rope 29 with a reel 30.

Operating the reel, for instance-by means of another motor, causes the rod 28 and the heater 4 to progress along the rod 1, for instance, in the direction of the arrow 7 for zone-melting the rodin the manner. described. During the processing, the spacing between the holders 2 and 3 can be varied for controlling'the cross section of the processed portion of material. Also-during the zonemelting, the motor 27 may be operated to keep the upper portion of rod 1 in revolution. Ifdesired, another motor may be provided for also revolving the holder 3 with the lower portion of rod 1.

A magnet coil 31 is mounted on rod 28 in concentric relation to heater ring 4 to produce during the melting operation a magnetic field to counteract the tendency of the molten materialto run off. For the same purpose the device is also equipped with the above-described nozzles 6 that, when in use, blow a stream of gas upwardly against the molten zone. The gas may be identical with the one used as a protective atmosphere. The nozzles are preferably mounted together with the heater so that they maintain, during the travel motion, their proper spacing from the molten zone.

The details of electromagnetic levitation devices can be readily obtained from prior publications. For such details, reference is made tothe U.S. patent to Wroughton et a1. 2,686,864 issued August 17, 1954, and to the prior patents cited by the Patent Oflice during the prosecution thereof.

I claim:

1. A crucible-free apparatus for purifying'an elongated rod-shaped body of meltable material of finite length, comprising means to clampingly support the ends of the body, heating means to heat a zone 'of said body to molten temperature, means for advancing said heating means along said body so that all portions within the processed length thereof are uniformly heated, the crosssectional area of said rod-shaped body being small enough so that surface tension can serve to contain said molten zone in position along said body, and extra electric means movable with said advancing means for producing a magnetic field at the surface of said melting zone effective to aid said surface tension in preventing molten material from running off.

2. The process of successively zone-melting and resolidifying an elongated body of fusible material which comprises supporting said body at both ends in a vertical position, applying heat to establish a molten zone extending across the cross section of the body, the axial length of said molten zone being such that the molten material is prevented from escaping by virtue of cohesive and adhesive forces, displacing said molten zone in an axial direction along said body, applying a moving magnetic field, independent from the heat source, at the surface of and moving with said displacing molten zone and directing a stream of heated gas upwardly against said molten zone, said field and said gas stream assisting said cohesive and adhesive forces in preventing escape of the molten material.

3. The process of successively zone-melting and resolidifying a rod of fusible material which comprises supporting said rod in a horizontal position at opposite end portions, applying heat to establish a molten segmental zone extending only part way across the cross section of the rod, the axial width and cross-sectional area of said molten zone being such that the amount of molten material is limited to that which is supportable solely by virtue of cohesive and adhesive forces, and relatively displacing said molten zone and said rod axially of said rod, While at the same time turning said rodso that substantially all surface portions thereof are progressively melted, the molten zone moving in a helical path in and along said body, directing a stream of cooling gas against the'nonmolten surface of the cross section which is heated to produce the molten zone, and displacing said stream of cooling gas together with, said molten zone.

4. The method of processing an elongated rod-shaped body of meltable material of finite length, comprising clampingly supporting the body in a horizontal position at opposite end portions, heating a segmental zone of said body to molten temperature, said zone extending only part way across said body, and advancing said heating along said body, the axial length and cross-sectional area of said molten zone being small enough so that surface tension alone can serve to contain said molten zone in position along said body, directing a stream of cooling gas against the non-molten surface of the cross section which is heated to produce the molten Zone, and displacing said stream of cooling gas together with, said molten zone.

References Cited by the Examiner UNITED STATES PATENTS 2,254,306 9/1941 Mott l48151 2,686,864 8/ 1954 Wroughton 22200.1 2,739,088 3/1956 Pfann -65 3,060,123 10/1962 Theuerer 75-65 OTHER REFERENCES Fitzgerald et al., Basic Electrical Engineering, 2nd edition, 1957, McGraW-Hill Book Co., N.Y., pages 168-172.

DAVID L. RECK, Primary Examiner.

WINSTON A. DOUGLAS, BENJAMIN HENKIN,

Examiners.

W. C. TOWNSEND, H. W. CUMMINGS, H. W. TAR- RING, Assistant Examiners, 

2. THE PROCESS OF SUCCESSIVELY ZONE-MELTING AND RESOLIDIFYING AN ELONGATED BODY OF FUSIBLE MATERIAL WHICH COMPRISES SUPPORTING SAID BODY AT BOTH ENDS IN A VERTICAL POSITION, APPLYING HEAT TO ESTABLISH A MOLTEN ZONE EXTENDING ACROSS THE CROSS SECTION OF THE BODY, THE AXIAL LENGTH OF SAID MOLTEN ZONE BEING SUCH THAT THE MOLTEN MATERIAL IS PREVENTED FROM ESCAPING BY VIRTUE OF COHESIVE AND ADHESIVE FORCES, DISPLACING SAID MOLTEN ZONE IN AN AXIAL 